Wednesday, August 12, 2009

Does the human skin sense temperature?

The answer is no, if you are in a hurry. But if you have time, let me pose a puzzle:

"Your body temperature is about 37C or 98F depending on which side of the Atlantic you are. If you are like most people it "feels" quite uncomfortable when the temperature outside is 37C. When asked most people say they like 20-27C (or 65-80F). "

Why?

The basic point is that our body is constantly producing heat. In fact, most of the food we eat is eventually converted to heat. This should explain why the average daily intake of an adult is approximately 2500 calories, while only about 300 of those are consumed by jogging for 30 minutes.

For heat to flow, it needs a temperature difference. Thus, the 10-12 celcius difference that a 25 celcius environment provides, enables us to efficiently dissipate the heat we constantly generate. This leads us to the answer.

Our skin senses heat flow (flux), not temperature.

There are several manifestations which illustrate this point:

When you sleep, you feel colder because your body processes have slowed down, producing less heat.

When it is hot your body sweats. Sweat vaporizes, and in doing so takes away a lot of heat from your body (aka latent heat of vaporization).

When it is windy you feel colder because heat is being convected away from your body faster (the wind chill effect).

Notice, how the outside temperature is only one of the factors which governs how "hot" or "cold" we feel. The mere fact that other things, such as level of activity, humidity, convection, perspiration affect that feeling suggests that we don't sense outside temperature.

5 comments:

Here's an even better analogy to help understand that the human body senses heat instead of temperature. Imagine you are fast asleep at night in a bedroom with a stone floor. However, next to the bed where you first put your feet when getting up, is a thick, plush rug. It's a cold winter's night, so the room was kept constant at a cool 60 deg F while you were sleeping.

When you wake up in the morning and step out of bed in your bare feet, we all know that if you step on the stone floor it will feel "cold". On the other hand, if you step on the rug, it will feel "warm". But wait: is the stone floor actually colder than the rug? Of course not; they are both at room temperature. So why does the stone floor feel colder?

Heat transferred by conduction depends on two things: temperature difference (the one with which most people are familiar) AND thermal conductivity (the one most don't understand). Newton first figured this out centuries ago and the resulting law, Newston's law of cooling, was named after him. So even though the temperature difference between your feet and either the stone floor or rug is the same, the thermal conductivity of the stone is much higher than that of the rug (which is an insulator). Hence, the stone floor "sucks" more heat from your feet than the rug, and your body senses this as a "cold" sensation. If your body truly sensed temperature, then the stone floor and rug would feel exactly the same when you stepped on them (since we know they are at the same temperature ... how could one have gotten hotter than the other during the night?!).

jd, i absolutely agree with your example. it is definitely more illustrative of the concept. in fact, i used a very similar example in a class test.

however, i would replace "thermal conductivity" in your explanation with a "heat-transfer coefficient". the former tells me something about the material (rug, floor, piece of metal), while the latter tells me about the material AND the interfacial heat transfer from the material to my skin.